1. Field of the Invention
The invention relates to a semiconductor device for optically coupling a semiconductor light-emitting device to an optical fiber.
2. Description of the Related Art
A lot of semiconductor devices for optically coupling a semiconductor light-emitting device to an optical fiber have been suggested. For instance, such semiconductor devices are suggested in Japanese Patent Publication No. 3-61927 (B2) (Japanese Unexamined Patent Publication No. 59-166906 (A)) and Japanese Unexamined Patent Publication No. 7-218773 (A).
Hereinbelow is explained a semiconductor device for optically coupling a semiconductor light-emitting device to an optical fiber, suggested in Japanese Patent Publication No. 3-61927, as an example.
FIG. 1 is a cross-sectional view of a semiconductor laser module including such a semiconductor device, disclosed in Japanese Patent Publication No. 3-61927, as an example.
The semiconductor device for optically coupling a semiconductor light-emitting device to an optical fiber is comprised of a spherical-lens holder 63 fixedly mounted on a stem 62 on which a heat sink 60 to which a semiconductor laser device 61 is fixed is mounted, a spherical lens 64 fit into a hole formed centrally with the spherical-lens holder 63 such that a center of the spherical lens 64 is on an extension of an optical axis of the semiconductor laser device 61, and wax 65 coated in a belt around the spherical-lens holder 63 to keep inside of the spherical-lens holder 63 in air-tight condition.
The spherical-lens holder 63 is comprised of a first portion 63 formed cylindrical, and a second portion 63a formed annular, formed integral with the first portion 63a at an upper end of the first portion 63a, and formed centrally with a hole into which the spherical lens 64 is fit.
The semiconductor device for optically coupling a semiconductor light-emitting device to an optical fiber cooperates with other parts to thereby define a semiconductor laser module. Positional relations between the semiconductor device and other parts are as follows.
On a base 66 on which the stem 62 is mounted is slidably mounted a ferrule holder 67 which surrounds the spherical-lens holder 63 therein. The ferrule holder 67 is formed with a projecting tube 67a into which a ferrule 69 to which an optical fiber 68 is physically connected is inserted.
The ferrule holder 67 is adjusted in position in a direction A perpendicular to an optical axis with the ferrule holder 67 being mounted on the base 66, and further, the ferrule 69 is slid in the projecting tube 67a in a direction B of an optical axis such that laser beams emitted from the semiconductor laser device 61 through the spherical lens 64 are focused on an end surface 68a of the optical fiber 68.
After optimizing an optical positional relation between the semiconductor laser device 61 and the optical fiber 68 through the spherical lens 64 in the above-mentioned manner, the ferrule holder 67 is fixed on the base 66 by resistance welding. In addition, the ferrule 69 is fixed to the ferrule holder 67 by adhesive 70.
Though the spherical lens 64 is supported on the spherical-lens holder 63 through the wax 65 in the conventional semiconductor device for optically coupling a semiconductor light-emitting device to an optical fiber, illustrated in FIG. 1, the spherical lens 64 is generally supported on the spherical-lens holder 63 through glass having a low fusing point, in order to more certainly keep the spherical lens 64 and the spherical-lens holder 63 hermetically sealed therebetween.
In general, the spherical-lens holder 63 is fixed to the stem by resistance welding. In such resistance welding, a temperature of about 1,400 degrees centigrade is applied instantaneously to the spherical-lens holder 63. Hence, a lower portion of the first portion 63a of the spherical-lens holder 63 is heated to a temperature of about 1,400 degrees centigrade, but since a temperature of about 1,400 degrees centigrade is applied to the spherical-lens holder 63 for quite a short period of time, an upper portion of the first portion 63a, for instance, a portion at which the first portion 63a and the second portion 63b are connected to each other is heated only to about 30 to 40 degrees centigrade.
Accordingly, the first portion 63a expands to a greater degree at a lower portion thereof than at an upper portion thereof. As a result of different degrees of expansion between the upper and lower portions of the first portion 63a, there is generated a thermal stress in the spherical-lens holder 63. As illustrated in FIG. 2, the thermal stress acts on the spherical-lens holder 63 as a compressive stress T directing towards a center from an outer edge of the spherical-lens holder 63.
The compressive stress T causes elastic deformation in the spherical-lens holder 63, as shown in FIG. 2 with a broken line 63c. 
In the semiconductor device illustrated in FIG. 1, an interface between the spherical lens 64 and the spherical-lens holder 63, in other words, the wax 65 or the glass has the smallest resistance to a stress among parts constituting the semiconductor device. Accordingly, when the spherical-lens holder 63 is elastically deformed as shown in FIG. 2 with the broken line 63c, a stress caused by the elastic deformation acts on the wax 65 or the glass most.
As a result, the wax 65 or the glass is cracked. If the crack reaches both inner and outer surfaces of the spherical-lens holder 63, there is generated a leakage path through which air enters from outside into inside of the spherical-lens holder 63. Thus, it is no longer possible to keep the spherical lens 64 and the spherical-lens holder 63 hermetically sealed therebetween, resulting in reduction in a light-emitting efficiency of the semiconductor laser device 61.